Coach is right. It's not common advice at the beginner level, still infrequent intermediate level but it's done, helps with flat (defend with power to the body) and topspin (make opponent volley low).
Most advice is geared towards lower levels, and people repeat it so even though you'll find many people asking to stick to continental, your coache's advice was sound, not just his "personal" thing. But it is more advanced, it introduces a grip change which is always challenging and also you need to select the ball you do it with (height, difficulty, reaction time). Grip changes can take a lot of time to get decent at.
Certain type of backhands can really benefit from the grip change but I'd say going for this now or not depends on your level, I can't give advice since I don't know you.
Since people seem to use youtube videos as reference I went to one I know gives (or used to give) pretty good technical instructions, and of course he talks about the backhand grip, you'll see he says something similar. Unfortunately, spanish only, no english captions: [https://www.youtube.com/watch?v=Dn3jGb8cQak](https://www.youtube.com/watch?v=Dn3jGb8cQak), around 5:15+